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Disclosure - Note 15 - Revenue from Contracts with Customers (Tables) Sheet http://www.bankfinancial.com/20221231/role/statement-note-15-revenue-from-contracts-with-customers-tables Note 15 - Revenue from Contracts with Customers (Tables) Tables http://www.bankfinancial.com/20221231/role/statement-note-15-revenue-from-contracts-with-customers 39 false false R40.htm 039 - Disclosure - Note 16 - Company Only Condensed Financial Information (Tables) Sheet http://www.bankfinancial.com/20221231/role/statement-note-16-company-only-condensed-financial-information-tables Note 16 - Company Only Condensed Financial Information (Tables) Tables http://www.bankfinancial.com/20221231/role/statement-note-16-company-only-condensed-financial-information- 40 false false R41.htm 040 - Disclosure - Note 1 - Summary of Significant Accounting Policies (Details Textual) Sheet http://www.bankfinancial.com/20221231/role/statement-note-1-summary-of-significant-accounting-policies-details-textual Note 1 - Summary of Significant Accounting Policies (Details Textual) Details 41 false false R42.htm 041 - 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Disclosure - Note 5 - Foreclosed Assets - Real Estate Acquired Through Foreclosure (Details) Sheet http://www.bankfinancial.com/20221231/role/statement-note-5-foreclosed-assets-real-estate-acquired-through-foreclosure-details Note 5 - Foreclosed Assets - Real Estate Acquired Through Foreclosure (Details) Details 55 false false R56.htm 055 - Disclosure - Note 5 - Foreclosed Assets- Roll Forward of Foreclosed Assets (Details) Sheet http://www.bankfinancial.com/20221231/role/statement-note-5-foreclosed-assets-roll-forward-of-foreclosed-assets-details Note 5 - Foreclosed Assets- Roll Forward of Foreclosed Assets (Details) Details 56 false false R57.htm 056 - Disclosure - Note 5 - Foreclosed Assets - Activity in the Valuation Allowance (Details) Sheet http://www.bankfinancial.com/20221231/role/statement-note-5-foreclosed-assets-activity-in-the-valuation-allowance-details Note 5 - Foreclosed Assets - Activity in the Valuation Allowance (Details) Details 57 false false R58.htm 057 - Disclosure - 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Note 10 - Income Taxes (Details Textual) Sheet http://www.bankfinancial.com/20221231/role/statement-note-10-income-taxes-details-textual Note 10 - Income Taxes (Details Textual) Details http://www.bankfinancial.com/20221231/role/statement-note-10-income-taxes-tables 69 false false R70.htm 069 - Disclosure - Note 10 - Income Taxes - Schedule of Income Tax Expense (Benefit) (Details) Sheet http://www.bankfinancial.com/20221231/role/statement-note-10-income-taxes-schedule-of-income-tax-expense-benefit-details Note 10 - Income Taxes - Schedule of Income Tax Expense (Benefit) (Details) Details 70 false false R71.htm 070 - Disclosure - Note 10 - Income Taxes - A Reconciliation of the Provision for Income Taxes (Details) Sheet http://www.bankfinancial.com/20221231/role/statement-note-10-income-taxes-a-reconciliation-of-the-provision-for-income-taxes-details Note 10 - Income Taxes - A Reconciliation of the Provision for Income Taxes (Details) Details 71 false false R72.htm 071 - Disclosure - Note 10 - 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